This invention relates in general to vehicle brakes and, in particular, to an improved composite disc brake rotor for use in a disc brake assembly, and to an improved casting method for producing such a rotor.
A typical disc brake rotor is formed from grey cast iron during a sand mold casting process. The rotor includes a generally hat-shaped body, and an outer annular section which are integrally cast as one-piece during the casting process. This kind of rotor is commonly referred to as a "full cast" rotor.
The hat-shaped body includes a mounting surface having a centrally locating pilot hole formed therein during casting, and a plurality of lug bolt receiving apertures equally spaced circumferentially about the pilot hole. The lug bolt receiving apertures are formed during a machining operation. The outer annular section, of the rotor includes two parallel outer surfaces which define a pair of brake friction surfaces. The brake friction surfaces can be cast as a single solid brake friction plate, or can be cast as a pair of brake friction plates disposed in a mutually spaced apart relationship by a plurality of ribs or fins to produce a vented rotor. In some instances, the rotor is formed with an integrally cast hub, and is referred to as a "uni-cast" rotor.
While grey cast iron rotors generally possess sufficient mechanical and thermal properties to satisfy requirements of disc brake systems, they are relatively heavy and, for passenger car and light truck applications, can each weigh up to approximately 30 pounds. Since rotors are considered rotating mass and unsprung mass as well as being part of the total mass of the vehicle, the weight of the rotor adversely affects the performance and fuel economy of a vehicle.
To produce a lightweight rotor, it has been suggested to cast the rotor from an aluminum alloy, such as 319 or 356 aluminum. However, while aluminum alloy rotors possess satisfactory thermal properties, they do not possess adequate mechanical properties of high temperature strength, hardness, and wear resistance, which are typically required for disc brake applications. In order to satisfy these mechanical properties and still produce a lightweight rotor, it is known to cast the rotor from an aluminum based metal matrix composite (MMC) containing silicon carbide particulate reinforcement. Such an aluminum MMC is commercially available under the name DURALCAN, a registered trademark of Alcan Aluminium Limited of San Diego, Calif.
The aluminum MMC provides the finished rotor with sufficient mechanical and thermal properties to satisfy the requirements of brake rotor designs at a significantly reduced weight. For example, it has been found that a weight reduction of approximately 60% over a comparable grey cast iron rotor can be achieved by casting the rotor from the aluminum MMC.
One disadvantage to castings made with the aluminum MMC is that they are rather expensive compared to the costs of castings made from grey cast iron and conventional aluminum alloys. Another disadvantage is that the very hard particulate reinforcement makes the aluminum MMC castings more difficult to machine compared to grey iron and conventional aluminum castings.
U.S. Pat. No. 5,183,632 to Kiuchi et al. discloses a method for producing an aluminum "composite" disc brake rotor in which only the friction plate portions are formed of a reinforced aluminum alloy, while the remainder of the rotor is an aluminum alloy. According to this method, an aluminum alloy is first cast or press molded to form a rough-shaped disc brake rotor body. Next, an annular recessed portion (corresponding to the friction plate portions) is formed in each rotor face by machining. A separate reinforced aluminum alloy powder preform or a mixture of an aluminum alloy powder and reinforcing particles is then placed in each of the recessed portions of the rotor. The rotor body including the preform or mixture is heated to a mashy state temperature, and then molded under pressure to secure the preform or mixture to the rotor body and produce a rough-shaped disc brake rotor.